1. Field of the Invention
The present invention relates to a reactor control rod and a method of manufacturing the same, and, more particularly, a long-life type reactor control rod suitable for a boiling water reactor and a method of manufacturing the same.
2. Description of the Related Art
Generally, a control rod of a boiling water reactor (abbreviated as a "BWR" hereinafter) is constructed by inserting neutron absorber elements made of a neutron absorbing material into a plurality of long sheaths. Each sheath has a deep U-shaped cross sectional shape. In this way, a plurality (four sheets) of wings are formed. A top end structure and a bottom end structure are secured to insertion top ends and insertion bottom ends respectively. U-shaped opening portions of the sheath are secured to the tie rod, so that the control rod is constructed to have a cruciform sectional shape.
In the BWR control rod in the related art, the sheath has been made of stainless steel. B.sub.4 C powder is filled into the stainless steel pipe having a diameter of about 5 mm. The stainless steel pipe with B.sub.4 C has been employed as a neutron absorber rod.
However, since boron (B) can react with the neutron to produce helium (He) and lithium (Li) and also has a short nuclear lifetime, the neutron absorbing capability of the neutron absorber rod is ready to deteriorate. In addition, since soundness of the neutron absorber rod is ready to deteriorate by increase of the partial pressure of He, etc., the mechanical lifetime of the neutron absorber rod is short.
Therefore, for the control rod which needs the long lifetime, a control rod, in which a part or all of the conventional neutron absorber rods are replaced with hafnium (Hf), has been developed and has been employed in practical use. The hafnium (Hf) can be used as a long-life type neutron absorbing material.
A specific gravity of Hf is very large such as about 13. Thus, if a Hf rod, which has the same cross sectional shape as the conventional neutron absorber rod using B.sub.4 C, is employed, the neutron absorbing capability (reactivity value) of the Hf rod is substantially identical to that of the conventional neutron absorber rod, nevertheless the weight of the entire control rod is increased up to about 1.5 times. For this reason, such Hf control rod fails to back-fit to the reactor in operation.
The inventors of the present invention have proposed the Hf control rod which has the so-called "trap type" configuration in which Hf is formed like a plate and then the core water is introduced into the clearance between two opposing Hf plates (refer to Japanese Patent Application Publication (KOKAI) Sho 57-80592, etc.). Further, taking into account a feature of the BWR that the neutron absorbing capability may be lowered at about half area on the insertion bottom end side, the inventors of the present invention have proposed the control rod having the structure in which an amount of Hf on the insertion bottom end side can be set less than that on the insertion top end side (refer to Japanese Patent Application Publication (KOKAI) Sho 62-235595, etc.).
The trap type long-life control rod using the Hf plates has already been employed in a great number of BWRs, and has achieved satisfactory actual results. In order to accumulate sufficient actual results as the control rod, the lifetime of the Hf control rod is set conservatively, i.e., shorter at the existing time, but such a prospect can be obtained up to now that the set lifetime can be prolonged still much more.
FIGS. 10A to 10C and FIGS. 11A and 11B are views showing outlines of design examples of a so-called Hf trap type control rod in the related art respectively. FIG. 10A is a perspective view, partially cut away, showing the Hf trap type control rod, FIG. 10B is a cross sectional view showing one wing of the Hf trap type control rod, and FIG. 10C is a perspective view showing a load supporting member ("load supporting spacer" or "top spacer") shown in FIG. 10B. FIG. 11A is a front view showing the Hf trap type control rod from which a front sheath is removed from FIG. 10A, and FIG. 11B is a view showing an example of distribution of thickness of the Hf plate, which is a neutron absorbing material fitted into the sheath, along the control rod axis direction (inserting/withdrawing direction, or sheath longitudinal direction).
The control rod 1 has a cruciform cross sectional shape. The top end structure 4 which is formed integrally with a handle 3 is secured to the insertion top end portion and the bottom end structure 5 is secured to the insertion bottom end portion. The cruciform tie rod 6 made of stainless steel is positioned at the axial center portion of the control rod 1. The opening portions of the stainless steel sheaths 7, each having a deep U-shaped cross sectional shape, are secured to respective projected portions of the tie rod 6 by welding, so that four sheets of wings 2 are constructed.
A plurality of water feed holes 9 are formed in the sheath 7 to enable flow of the core water.
Two sheets of Hf plates 10 are arranged oppositely and spaced from each other by load supporting spacers (top spacers) 12 in the sheath 7, so as to create a water gap 11 (clearance which is filled with the water when used in the reactor) therebetween. The top spacer 12 has a top-like structure. The top spacer 12 comprises an interval holding portion (spacer portion) 12a whose thickness of the body portion has a spacer function, and axes 12b. The axes 12b of the top spacers 12 are inserted into holes of the sheath 7, and then welded to the sheath 7 to support the weights of the Hf plates 10.
The stainless steel sheath 7 and the Hf plate are different three times in thermal expansion coefficient. Therefore, a diameter of the hole in the Hf plate 10, into which the axis 12b of the load supporting spacer 12 is inserted, is set larger than that of the axis 12b of the load supporting spacer 12 to thus avoid the problem of thermal expansion/contraction in the thermal cycle.
In the example in FIG. 11A, the Hf plate 10 is divided into a plurality of pieces, e.g., eight pieces along the axis direction (sheath longitudinal direction) of the control rod 1. Respective pairs of Hf plates can be held by four top spacers 12. As shown in FIG. 11B, the pairs of Hf plates are formed thicker toward the insertion top end and formed thinner toward the insertion bottom end. This is because the neutron irradiation amount must be increased at the insertion top end and thus the reactivity value (reactivity effect) must be set higher. Normally, lengths of the Hf plates 10 in the axis direction (sheath longitudinal direction) are set identically along the axis direction, and thicknesses of the sheath plates are kept constant along the axis direction.
It has been disclosed in "Reactor Material Handbook" (published by The Nikkan Kogyo Shimbun Ltd.), etc. that the thermal expansion coefficient (17.8.times.10.sup.-6 /deg-C) of stainless steel is about three times larger than that (5.9.times.10.sup.-6 /deg-C) of Hf.
In the meanwhile, both the stainless steel and the Hf are very excellent in the high temperature corrosion resistance. However, if the longer set lifetime is needed, it is preferable that the well known electrochemical corrosion problem caused by adjacent metals of a different kind, i.e., the stainless steel sheath and the Hf plate should be thought over and thus the countermeasure should be applied positively.
More particularly, in the case of Hf, an oxide film is formed on the surface of the Hf plate in the high temperature core water, and therefore a so-called "passive state oxide film" to protect the inside of Hf is formed. However, such oxide film is deteriorated in mechanical strength comparing with the Hf metal or the Hf alloy. Hence, it is desired that the structural design not to apply a high friction force to the oxide film should be adopted and also the electrochemical problem should be relaxed by improving the water feed characteristic.